1. Field of the Invention
The present invention relates to a process for preparing aliphatic and cycloaliphatic oximes obtained by reacting in the liquid phase the corresponding saturated primary aliphatic or cycloaliphatic amine with oxygen in the presence of catalysts based on compounds of metals of Group IV B of the Periodic System (titanium, zirconium, hafnium).
The (cyclo)aliphatic oximes so obtained are products, which have "per se" useful applications, for example as oxidation inhibitors, or they represent interesting intermediates for the production of amides with corresponding wide applicative possibilities.
In particular, when the amine is cyclohexylamine, the resulting cyclohexanone oxime can be further converted to the corresponding cyclic amine, or epsilon-caprolactam.
The caprolactam represents the starting monomer for obtaining nylon 6.
2. Discussion of the Prior Art
Already known is the oxidation reaction of saturated aliphatic or cycloaliphatic primary amines (cyclohexyl-amine), having a C--H linkage in alpha position with respect to the aminic group, with hydrogen peroxide to obtain oximes, by operating in the presence of catalysts based on molybdenum, tungsten, uranium, or with organic hydroperoxides, by operating in the presence of catalysts based on titanium, molybdenum, tungsten and vanadium in the organic phase.
Nevertheless, the use of the abovesaid oxidizing agents is expensive and, in the case of the utilization of an organic hydroperoxide, further complications connected with the separation, purification and the like can occur due to the presence, in the final reaction mixture, of by-products (alcohols) deriving from the reduction of the hydroperoxide.
To these drawbacks, the known operative risks related to the handling of peroxide compounds are to be added.
On the other hand it is also known to prepare oximes from saturated (cyclo)aliphatic primary amines of the abovesaid type by using oxygen as an oxidant in the presence of solid catalysts based on SiO.sub.2 gel, gamma Al.sub.2 O.sub.3, optionally associated with tungsten oxides, operating in the gas phase.
This is a process which involves relatively severe operative conditions due to the gas phase (temperatures approximately ranging from 120.degree. C. to 250.degree. C., etc.).
On the other hand it was assumed so far that metals such as titanium should generally be only efficient activators of peroxide compounds.